1. Technical Field
The present invention relates to optical coupling systems for coupling semiconductor laser with optical fiber and in particular relates to methods for diode-laser beam shaping for construction of high-efficiency and high-power solid state lasers including fiber lasers, and for high efficiency transmission of diode laser power through optical fiber.
2. Background Art
The development of diode pumped fiber lasers has been rather successful recently. The scaling of various physical effects has greatly benefited this development. Diode lasers can provide concentrated pumping energy and thus enhance the efficiency of fiber lasers. The long thin geometry of fiber also makes heat removal much easier than in bulk solid state lasers. In end-pumped fiber lasers a large outer cladding is used in cladding pumping. Pump light, often piped through fibers from pump lasers, enters the outer core, where it is confined so that it passes through the inner core, exciting the laser species. Stimulated emission from the laser species remains in the inner core. By converting the low brightness beam from the pump diode bar into a tighter beam pumping a fiber laser can multiply brightness by a factor of more than 1000. By using such configuration, Polaroid, for example, reported a diode cladding pumping fiber laser reaching 35 W.
Currently, a typical fiber laser device includes a tens-of-meters double clad silica fiber with a small diameter and small NA core doped with active species, centered within a much larger inner cladding, surrounded by a soft low index fluoropolymer providing an acceptance NA of 0.45 for pump radiation. Pumping laser beams from laser diodes are coupled into the fiber inner cladding through the dichroic end mirror. (HR laser, HT pump). Among other things, the cladding pumping geometry and the coupling efficiency of pumping laser diode array are the main factors under intense research. Proper geometry is essential for increasing the efficiency of cladding pumping. Good method of coupling will allow more diode laser power to be injected into the fiber laser, which in turn also increase the efficiency of a fiber laser.
A typical high-power laser diode array (LDA) has an a broad area light emitting aperture (1 cmxc3x971 xcexcm) comprising light emitting elements which are multiple spaced apart segments. In one typical commercial LDA product, for example, each segment has a width less than 200 xcexcm, and may be divided into 20 sub-segments. Each sub-segment has an aperture width of 3-6 xcexcm, and emits about 30 mW-60 mW. The effective aperture size in the transverse direction perpendicular to the plane of laser active region (the fast axis) is about 1 xcexcm. Typical fast axis divergence is 30-40 degree and slow axis divergence is 10-15 degree. A typical high-power LDA can deliver 20 W laser power. Those more powerful can deliver 40 W or 60 W with this geometry. By using diode array stacks, however, 500-1400 W can be obtained. Because of the elongated geometry of LDA, it has been always a challenge to couple or inject high power (such as 4000 W) into a fiber cladding aperture (such as an aperture of 200 xcexcmxc3x97500 xcexcm, NA 0.45).
In order to send more power into optical fiber, many efforts have been made to concentrate light from diode laser arrays. There are a number of patents dealing with concentrating multiple emitter laser diode beams, such as U.S. Pat. Nos. 5,887,096, 5,825,551, 5,808,323, 5,805,748, 5,513,201, and PCT WO99/35724, and PCT97/14073. Some other disclosures are shown in U.S. Pat. Nos. 5,802,092, 5,793,783, 5,790,310, 5,594,752, 5,579,422, 5,568,577, 5,333,077, 5,185,758, 5,139,609, and 4,428,647.
In U.S. Pat. No. 5,887,096, an arrangement in which a reflection lens system shapes and guides beams from a rectilinear laser diode array with beam outlet surfaces lying in a common plane is disclosed. In order to map the beams from the individual laser diodes to form a substantially uniform radiation field or pattern using such an arrangement, at least one first reflection component having a reflection surface is associated with each beam in order to bring together the individual outlet beams. The reflection surfaces are disposed in mutually offset planes, the offset corresponding sequentially to the sequence of the laser diodes in the array. And it has been found advantageous for at least one second, additional reflecting surface to be assigned to each first reflecting surface and placed in the beam path of each beam emerging from a laser diode. In U.S. Pat. Nos. 5,825,551, 5,808,323, 5,805,748, different laser beam shaping systems are disclosed. With these methods, an elongated laser beam is divided into a plurality of beam sections that is then rearranged into a more circular cross-section more suitable for pumping. Thus, beam dividing and rearrangement is achieved in these Patents using two parallel mirrors, using multiple small mirrors, or using multiple refractive parallel plates, respectively. In PCT WO97/14073, Hollemann et al shows a device for combining and shaping the radiation from several laser diode cells consisting of at least two laser diodes. For the systems discussed in the above disclosures, the requirements in system alignment can be difficult to meet in practice. In U.S. Pat. No. 5,805,323, a single row of mirrors is also used to shape the beams from a laser diode array to a parallelogram-shaped laser beam bundle. But the mirror is difficult to realize in practice. In U.S. Pat. No. 5,513,201, an optical-path rotating device comprising a group of complicated prisms is disclosed. In PCT 99/35724 and in an earlier publication (SPIE Proceedings Vol. 3,008,202, 1997), Goring et al disclosed an optical system for symmetrizing the beam of one or more superimposed high-power diode laser by using refraction components. A common feature of the prior art is that one reflection component is associated with each beam from the diode laser array. This leads to low flexibility and difficulty in manufacturing. In U.S. Pat. No. 4,428,647, Spragne et al disclose systems in which each laser emitter of a diode laser array has its own lens mount adjacent to it in the space between the laser array and objective lens of the system. The purpose of the lens array is to change the angle of divergence of light beams leaving the emitting surface of the laser array at the slow axis so that the light beam can be collected efficiently by the objective lens. In U.S. Pat. No. 5,185,758, Fan et al describe a method for scaling a pumped medium to higher power with multiple light source. The output beam of each light source is substantially collimated by respective collimating optics, and the beams of sources are substantially parallel to each other after collimation. An optical system is provided to focus the collimated and parallel beams. The methods described in U.S. Pat. Nos. 5,802,092, 5,793,783, 5,790,310, 5,594,752, 5,579,422, 5,568,577, 5,333,077, and 5,139,609 are similar to the methods mentioned above. However, since lens arrays can only collimate the beam from the diode array to a limited extent, obvious divergence still exists. Because of the beam divergence, laser diode arrays must be close to optical fiber so that the beam spot is small enough to achieve effective coupling. When multiple laser diode arrays are combined, the dimension of beam spot on the fiber aperture plane becomes larger due to the increased distance between laser diode arrays and the fiber aperture. As a result, these methods can not efficiently combine the beams from a plurality of diode laser arrays in to an optical fiber. For example, with these methods, it is impossible to effectively couple the beams from 200 pieces of 20 W diode laser bars into a fiber to make a high-power fiber laser.
Accordingly, it is the principal object of the present invention to provide methods and devices to shape and combine the beams from laser diode arrays or stacks so that high-efficiency and high-power coupling with target, such as optical fiber, can be achieved. High-efficiency and high-power solid state lasers such as fiber lasers, and high-efficiency transmission of diode laser power through optical fiber can be achieved.
It is another object of the present invention to teach methods of dividing and rearranging diode laser beam into a more circular laser beam.
It is also an object of the present invention to provide a method and apparatus with which a laser beam from a laser diode array with collimating structures can be transmitted for a long distance while the beam spot dimension can be kept substantially unchanged even if the beam from said array is not substantially collimated.
It is another object of the present invention to provide brightness conserved optical pumping systems.
It is another object of the present invention to teach methods of coupling high laser power coupling into optical fiber with high efficiency, such as a method of injecting at least 4000 W power into 0.5xc3x970.2 mm2 cladding and generating fiber laser output of 2500 W.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, a preferred apparatus of this invention may comprise a laser fiber with inner cladding, a plurality of brightness substantially-conserved laser diode array modules, and a brightness substantially-conserved optical system for coupling the beam from said laser diode array module into said inner cladding, with each of said laser diode array modules containing a beam shaping structure.
Additional objects, new features and advantages of the present invention will be set forth in part in the following description. Further scope of applicability of the present invention will become apparent from the detail description of the invention provided hereinafter. It should be understood, however, that the detailed description of the invention and the specific examples presented, while indicating preferred embodiment of present invention, are provided for illustration purposes only, because various changes and modifications within the scope and spirit of the present invention will become apparent to those of ordinary skill in the art from the detail description of the invention that follows.